1. Field of the Invention
The present invention relates to telephone receiver circuits, and in particular, to telephone receiver circuits providing sidetone signal generation for feedback during voice transmission.
2. Description of the Related Art
Telephonic communication within a noisy background environment can be quite difficult due to the background noise. This problem is worsened by the background noise picked up by the microphone and fed back to the speaker by way of a sidetone signal, thereby causing the background noise to be mixed with the received audio signal and received at both ears of the user. While audio feedback via the sidetone signal is very important in helping the user talk in a normal manner on the telephone handset, it is nonetheless troublesome when trying to communicate with a high level of background noise.
Referring to FIG. 1, a basic telephone transceiver system 10 incorporates the following basic functional elements: transmitter circuitry 20; a voice activity detector (VAD) 22; a sidetone signal generator 24; receiver circuitry 26; a signal duplexor 28; a signal combiner 30; an antenna 32; a microphone 34; and a speaker 36; all interconnected substantially as shown. As discussed further below, a second microphone 38 may also be included.
In accordance with well known principles, the system operates as follows. During signal reception, an incoming high frequency (e.g., radio frequency) wireless signal 11 is received by the antenna 32 and converted to a corresponding high frequency electrical signal 33. The signal duplexor 28 routes the incoming signal 29 to the receiver circuitry 26 for processing in accordance with well known principles (e.g., frequency down-conversion, filtering and demodulation). The resulting audio signal 27 is combined with a sidetone signal 25 in a signal combiner 30 (e.g., a signal summing circuit), and the resulting audio signal 31 is provided to the speaker 36 which converts it to the corresponding sound 37. (During signal reception, the sidetone signal 25 is substantially inactive or of zero magnitude.)
During signal transmission, incoming sound 15 generated by the voice of the user is picked up by the microphone and converted to an audio signal 35 which is distributed to the transmitter circuitry 20, VAD 22 and sidetone generator 24. This audio signal 35 is processed by the transmitter circuitry 20 in accordance with well known principles (e.g., modulated onto a higher frequency signal, frequency up-converted and filtered) to produce a high frequency signal 21 for transmission. The signal duplexor 28 routes this signal 21 to the antenna 32 for transmission as a high frequency (e.g., radio frequency) wireless signal 13.
The VAD 22 monitors the incoming audio signal 35 for voice activity and provides a control signal 23 to the transmitter circuitry 20 for performing voice-operated transmission (VOX). During VOX operation, the presence of voice activity within the incoming audio signal 35 causes the VAD 22 to provide the control signal 23 in such a manner that the transmitter circuitry 20 is enabled (or otherwise controlled) to provide the high frequency output signal 21 for transmission. In the absence of voice activity within the audio signal 35, the VAD control signal 23 causes the transmitter circuitry 20 to be disabled inasmuch as at least the power amplifier for the output signal 21 is disabled or turned off. This allows electrical power to be conserved, which is particularly important with the system 10 is being powered by a portable battery.
Also during signal transmission, the audio signal 35 is processed by the sidetone generator 24 (e.g., attenuated) to produce a sidetone signal 25 which is provided as one of the signal components in the speaker signal 31 for conversion to feedback sound 37 for the user, as discussed above.
However, as noted above, this feedback sound 37, when it contains background noise, can pose problems for the user when trying to communicate in a noisy environment. One conventional technique which has been used to address this problem is to use a second microphone 38 which picks up the background noise 17 and converts it to a “noise” audio signal 39 which is processed within the sidetone generator 24 to determine what level of sidetone signal 25 should be provided for conveyance to the speaker 36. For example, when the background noise 17 is sufficiently high, the “noise” signal 39 will be above a predetermined threshold which causes the sidetone generator 24 to either disable the sidetone signal 25 or set it at some minimum magnitude.
While this technique can be somewhat effective, it requires the use of a second microphone 38 and, therefore, results in a higher-cost system 10. Additionally, the nature of the background noise 17 can make it problematic in establishing a reliable threshold within the sidetone generator 24 for determining when the audio sidetone signal 25 should be provided. Even if the second microphone 38 is avoided and the primary incoming audio signal 35 is used for tracking the background noise, the problem of establishing the appropriate threshold, as noted, still remains.